This invention relates to microwave amplifier circuits having a compact circuit configuration which may be fabricated with, for example, gallium arsenide monolithic microwave integrated circuit (MMIC) technology and which may have such good coupling at both input and output terminals (for example, over a bandwidth in excess of an octave) that several circuits of this type can be cascaded without undesirable interactions.
As MMIC technology advances and the application of MMICs in microwave systems becomes more widespread, there is strong pressure to reduce costs by increasing circuit density. The more circuits that can be accommodated on a single MMIC substrate, whether they be of the same or different functionality, the lower will be the material costs of the GaAs and the cost of subsequent assembly. An important factor in the successful integration of several circuits on a single substrate is the quality of the impedance match at input and output terminals. To ensure that the circuits can be cascaded on the substrate without unacceptable interactions, each circuit should have good input and output reflection coefficients.
There are at least two different traditional approaches to matching, namely: (1) the use of reactively matched networks to form both input and output coupling circuits, and (2) the adoption of a distributed amplifier configuration to provide both input and output matching.
Particular examples of approach (1) are described in the paper "Optimum Gain-Bandwidth Limitations of Transistor Amplifiers as Reactively Constrained Active Two-Port Networks" by W. H. Ku et al. in IEEE Trans. Circuits and Systems, Vol. CAS-22, pages 523 to 533 (June 1975), and the paper "Design and Performance of Microwave Amplifiers with GaAs Schottky-Gate Field-Effect Transistors" by C. A. Liechti et al. in IEEE Trans. Microwave Theory and Tech. Vol. MTT-22, pages 510 to 517 (May 1974); the whole contents of both these publications are hereby incorporated herein as reference material. A compact circuit configuration results from the use of reactively matched input and output coupling circuits for a single amplifier device (e.g. a GaAs Schottky-gate field-effect transistor, commonly termed "MESFET"). In the Liechti et al. paper, the output matching is modified to provide frequency compensation. However, such reactively-matched circuits generally have inferior wideband amplifier characteristics and inferior matching, as compared with distributed amplifiers.
Distributed amplifiers are wideband, provide flat gain, have excellent input and output reflection coefficients (S11 and S22) and have high tolerance to inaccuracies in circuit modelling or to variations in the fabrication process. Particular examples of the distributed amplifier approach are described in United States patent specifications U.S. Pat. No. 4,853,649, U.S. Pat. No. 4,486,719, U.S. Pat. No. 4,595,881, and U.S. Pat. No. 4,876,516, Proceedings Letters of IEEE (June 1969) pages 1195 to 1196 on "A MESFET Distributed Amplifier with 2 GHz Bandwidth" by W. Jutzi, and IEE (GB) Digest No. 1989/75 of the Colloquium "Multi-Octave Active and Passive Components and Antennas" pages 4/1 to 4/5 on "Designing Distributed Amplifiers for Prescribed Gain Slope" by the present inventor. The whole contents of all these publications are hereby incorporated herein as reference material.
FIG. 1 of U.S. Pat. No. 4,853,649 is a typical drawing of the equivalent circuit of a distributed amplifier comprising four field-effect transistors (FETs) 3 connected between two artificial transmission lines. The lines are formed by low-pass filter networks of inductors 7 and 8 and are terminated at each end by resistive loads. An input signal injected into the gate transmission line 7, propagates as a travelling wave along the line. This travelling wave develops a voltage on the gates 4 of the FETs 3 before being dissipated in the terminating load 9. The FETs 3 respond by translating the voltage excitations into currents which are then injected into the drain transmission line 8. Half the currents propagate in one direction and half propagate in the opposite direction; but the phases of these currents are such that those propagating towards the internal load 10 cancel, whilst those propagating towards the output 2 add together to produce the output signal. To ensure the correct phasing of the currents, the gate and drain artificial transmission lines 7 and 8 must be electrically identical. As lowpass filters, they must therefore have the same cut-off frequency f.sub.c. The cut-off frequency also determines the operating frequency bandwidth of the amplifier. FIG. 1 of U.S. Pat. No. 4,853,649 illustrates a single amplifier stage, and several of these circuits (each comprising four FETs) can be cascaded together.
For simplicity, the artificial transmission lines of a distributed amplifier are often represented as a cascade of lowpass filter sections with constant K, for example as illustrated in FIG. 3a of U.S. Pat. No. 4,853,649. K.sup.2 is the product of shunt and series impedances of the line. Using such an analysis of the lines, the following expressions for the characteristic impedance Zo and upper cut-off frequency f.sub.c can be obtained in terms of L and C, where L and C are the respective inductance and capacitance per unit cell.
(Zo).sup.2 =L/C . . . E(1) PA1 f.sub.c =(.pi..multidot.Zo.multidot.C).sup.-1 . . . E(2) PA1 (n.sup.2 .multidot.gm.sup.2 .multidot.Rs, R1.)/4 . . . E(3) PA1 n is the number of FETs, PA1 Rs is the signal-source impedance,
If ohmic losses are ignored, an approximate expression for the power gain G of a distributed amplifier is:
where gm is the mutual conductance of an individual FET,
and R1 is the load resistance.
Rs and R1 are also equal to the characteristic impedances of the gate and drain lines. The expression E(3) is useful for indicating the effects of changes in gm or line impedance. However E(3) must be used with caution since, in the absence of losses, it implies there is no limit to the gain that can be achieved just by adding more FETs. In fact, the maximum achievable gain (MAG) is always less that the MAG of the individual FETs.
Despite its well known advantages, not least of which is its ability to be cascaded, the distributed amplifier suffers from two basic disadvantages. Firstly, it is larger than might be desirable for an individual gain stage. Secondly, half the output current of the FETs is wasted by travelling in the wrong direction on the drain line. The waste of current causes losses in efficiency and gain.